Hepatitis delta virus (HDV) is an important human pathogen that causes acute and chronic liver disease. It infects individuals who are infected with hepatitis B virus. The long-term objectives are to determine the structure of the HDV RNP and its specific roles in virus replication and packaging. We will use a combination of in vitro methods, particularly a newly developed highly specific assay for the formation of the viral ribonucleoprotein (RNP) complex, as well as complementary experiments in cultured cells, to identify critical structures in the protein and the RNA and determine their relationships to viral replication. Understanding these relationships will lead to the development of a model of RNP assembly for this structurally minimal virus that takes into account protein-protein and protein-nucleic acid interactions and will influence our understanding of larger, more complex viruses. The specific aims are: 1) Characterize the structure of the HDAg-HDV RNA ribonucleoprotein complex assembled in vitro;2) Determine the relationship between the structure of the RNP formed in vitro and its function in cells. PUBLIC HEALTH RELEVANCE: Hepatitis delta virus (HDV) is a unique human pathogen that causes severe liver disease. The long-term objectives of this proposal are to characterize the large macromolecular complex formed by the virus protein and RNA. Understanding the structure of this complex and how it is assembled will provide a basis for advancing our understanding of HDV replication and pathogenesis, as well as other viruses.